Specification for Super 3G is defined as LTE (Long Term Evolution) by the standardization group 3GPP (3rd Generation Partnership Project) for W-CDMA (Wideband-Code Division Multiple Access). The Super 3G is a standard further developed from an extended technology HSPA (High Speed Packet Access) of the W-CDMA. The Super 3G realizes fast communication of higher than 100 Mbps in downlink and higher than 50 Mbps in uplink and is designed to improve latency and enhance frequency utilization.
In the LTE, an OFDMA (Orthogonal Frequency Division Multiple Access) scheme is used as a downlink radio access scheme, and a SC-FDMA (Single-Carrier Frequency Division Multiple Access) scheme is used as an uplink radio access scheme. (See non-patent document 1, for example.)
The OFDMA scheme is a multi-carrier transmission scheme where a frequency band is segmented into multiple smaller frequency bands (subcarriers) and data is carried in the individual subcarriers for transmission. The OFDMA scheme realizes fast transmission by densely arranging the subcarriers on a frequency axis orthogonally and accordingly can improve frequency utilization.
The SC-FDMA scheme is a single-carrier transmission scheme where a frequency band is segmented for individual terminals and the terminals use the different frequency bands for transmission. The SC-FDMA scheme can reduce interference between the terminals easily and effectively as well as achieve smaller variations of transmit power. Accordingly, the SC-FDMA scheme is preferred from the viewpoints of less power consumption and broader coverage for the terminals.
Furthermore, discussion of an LTE-Advanced system, which is developed from LTE, has been undertaken. The same radio access schemes are used for the LTE system and the LTE-Advanced system. Also in the LTE-Advanced system, the OFDMA scheme is used as the downlink radio access scheme.
In the LTE system and the LTE-Advanced system, one or more resource blocks (RBs) or resource units (RUs) are assigned to user equipments in downlink and uplink for communication.
In the LTE system, the downlink resource blocks are represented in a temporal domain and a frequency domain. The resource blocks are represented in a resource grid consisting of NRBDLNSCRB subcarriers and NsymbDL OFDM symbols. NRBDL denotes a downlink bandwidth represented by the multiple NSCRBs and ranges from 6 to 110 (6≦NRBDL≦110). NSCRB denotes a resource block size of a frequency domain represented by multiple subcarriers. NsymbDL denotes the number of OFDM symbols within a downlink slot. For example, the resolution of the temporal domain may be equal to 1 ms, and the resolution of the frequency domain may be equal to 180 kHz.
Also in the LTE system, the uplink resource units are represented in a temporal domain and a frequency domain similar to the downlink. The resource units are represented in a resource grid consisting of RBULNSCRB subcarriers and NsymbUL SC-FDMA symbols. NRBUL denotes a uplink bandwidth represented by the multiple NSCRBs and ranges from 6 to 110 (6≦NRBUL≦110). NSCRB denotes a resource block size of a frequency domain represented by multiple subcarriers. NsymbUL denotes the number of SC-FDMA symbols within an uplink slot. For example, the resolution of the temporal domain may be equal to 1 ms, and the resolution of the frequency domain may be equal to 180 kHz.
For convenience, the resource block and the resource unit are synonymously used, and either of them represents unit of resource assignment. The resource blocks are shared among a large number of user equipments or user apparatuses within a system. As one example, one resource block has a bandwidth of 180 kHz. For example, a system band of 5 MHz would include 25 resource blocks. A base station apparatus determines which user equipment the resource blocks are assigned to in each subframe of 1 ms in the LTE, for example. The subframe may be referred to as a TTI (Transmission Time Interval). The assignment of radio resources is referred to as scheduling.
In downlink, the base station apparatus transmits a shared channel to the selected user equipment in one or more resource blocks. The shared channel is referred to as a PDSCH (Physical Downlink Shared CHannel).
In uplink, a scheduled user equipment transmits a shared channel to the base station apparatus in one or more resource blocks. The shared channel is referred to as a PUSCH (Physical Uplink Shared Channel).
In a communication system using the shared channel, it is necessary to signal (indicate) to which user equipment the shared channel is assigned for each subframe. A control channel for use in this signaling is referred to as a PDCCH (Physical Downlink Control Channel) or a DL-L1/L2 control channel. In addition to the PDCCH, the downlink control signal may be transmitted in a PCFICH (Physical Control Format indicator Channel), a PHICH (Physical Hybrid ARQ Indicator Channel) and so on.
In the LTE and the LTE-Advanced, one-cell repetition is applied. In one-cell repetition, the same frequency channel is available in adjacent cells. In the LTE and the LTE-Advanced systems, the OFDMA scheme is used in downlink, and accordingly users are orthogonal each other in a cell. In other words, the same frequency channel is used in adjacent cells, and if a common subcarrier is used in the adjacent cells, the adjacent cells will cause interference each other. Particularly, signals that a user equipment residing in a cell edge (boundary) receives from a camped base station are susceptible to interference caused by signals transmitted from another base station adjacent to the camped base station. The interference caused by the signals transmitted from the adjacent base station would degrade throughput.
As one approach for improving the above-mentioned throughput degradation for the user residing in the cell edge, interference coordination by FFR (Fractional Frequency Reuse) is proposed.
In the FFR, a frequency available in the cell edge is reserved for each cell beforehand. In other words, in the FFR, different frequency bands for the cell edge area and the other areas are made available. In the FFR, a system band is segmented into multiple frequency bands. Different ones of the frequency bands resulting from the segmentation of the system band are assigned to user equipments residing in the cell edge area of a certain cell and user equipments residing in the cell edge area of another cell adjacent to the certain cell.
FIG. 1 illustrates one exemplary assignment of frequency bands in accordance with the FFR. In FIG. 1, the horizontal axis indicates the frequency, and the vertical axis indicates transmit power of a base station.
FIG. 1 illustrates an example of the FFR where three-cells repetition is applied to a user residing in the cell edge. In the illustrated example, the system band is segmented into three frequency bands. Hereinafter, the frequency bands resulting from the segmentation of the system band are referred to as segmented frequency bands. Frequency bands including common segmented frequency bands are assigned to user equipments residing in areas other than the respective cell edge areas of cells 1, 2 and 3. On the other hand, a segmented frequency band is assigned to user equipments residing in the cell edge areas of the cells such that the segmented frequency band cannot overlap with other segmented frequency bands for use in the cell edge areas of the adjacent cells. Furthermore, the transmit power is controlled such that higher transmit power is assigned to the user equipments residing in the cell edge area than to the user equipments residing in areas other than the cell edge area.
By assigning mutually different frequency bands to the user equipments residing in the cell edge area in the cell and the user equipments residing in the cell edge area in the adjacent cell and increasing the transmit power, the user equipments residing in the cell edge areas could avoid the inter-cell interference. The avoidance of the inter-cell interference can improve the throughput of the user equipments residing in the cell edge areas.
Meanwhile, in resource block scheduling, a metric Mu,f(i) is calculated for each user, each resource block and each subframe, and the calculated metric values are compared. Herein, i indicates a temporal element (for example, a subframe), u indicates a user (user index), and f indicates a resource blocs (frequency). The metric is an indicator indicative of priority for assigning resource blocks to user equipments. The metric is used for purpose of scheduling at a base station apparatus. The resource blocks are assigned in priority to a user equipment having a larger metric value.
Also, a transmission format (a data modulation scheme and a channel coding rate (or a data modulation scheme and a data size)) for the assigned resource blocks is determined based on a channel condition. Generally, the metric includes a quantity indicative of the channel condition (such as SINR (Signal-to-Interference plus Noise power Ratio)) for the scheduling. Various metrics may be used depending on implementations of the scheduling. For example, a maximum CI method may be used to enhance the system throughput, and the metric Mu,f(i) is given as follows,Mu,f(i)=γu,f(i).
The right-hand side indicates an instantaneous reception SINR at subframe i for user equipment u. For convenience, uplink scheduling is assumed. Since a user equipment having a better channel condition always communicates, the throughput could be maximized. However, fairness among users might be impaired. For this reason, a scheduling method called a PF (Proportional Fairness) method may be used.
In the PF method, the following metric is used,Mu,f(i)=γu,f(i)/E(γu,f),where E means averaging. For example, E may be an average data rate and be found as a temporal average over about 100 ms to 1 second. In this case, it means that although the average may be influenced by shadowing or distance variations, influence by instantaneous fading is smoothed.
It is similar to the maximum CI method in that resource blocks are assigned to a user equipment having a greater metric Mu,f(i). However, it differs from the maximum CI method in that an average value of reception quality for that user equipment is additionally considered. In this method, when the channel condition of individual user equipments exceeds the average channel condition of the user equipments, resource blocks are assigned. Thus, it is possible to improve the throughput as well as address unfairness caused by the maximum CI method.